Owing to the progress of apparatuses for recording and reproducing digital image data such as a digital VTR or a DVD (Digital Versatile Disk), a number of digital moving images which can be reproduced by these apparatuses are now provided. Also, various digital moving images are distributed via a digital television broadcast via an internet, a broadcasting satellite, or a communication satellite and users are becoming able to use high-quality digital moving images.
The digital moving image is able to be copied at high-quality easily at a digital signal level, and has a possibility to be copied limitlessly unless it is protected by some copy prohibition or copy control measures. Therefore, in order to prevent a false copy of the digital moving image or to control the number of generation of copy by authorized users, there is contemplated a method of adding information for controlling the copy to the digital moving image, preventing false copy using the added information, and limiting the copy.
As a technology to superimpose separate additional information on the digital moving image as described above, a digital watermark is known. The digital watermark is a technology to embed information such as a copyright of a content, identification information of users, right information of copy right holder, conditions of usage of contents, confidential information required when it is used, and copy control information as described above (hereinafter, referred to as watermark information) in the digitalized contents such as sounds, music, moving images, and still images in a hardly recognizable manner, and detecting the watermark information from the contents as needed to achieve copyright protection including usage control and copy control and promotion of secondary usage.
A typical example of the false copy is DVDs of pirated edition. Many of the DVDs of pirated edition are made by audiences by shooting an image projected on a screen by a video camera in a theater and generating DVDs from the shot image. In order to prevent the false copy of this type, copy or reproduction control information or an image ID is embedded in a movie film in advance as a digital watermark, and the digital watermark information (control information or the image ID) embedded therein is detected from the image shot by the video camera, thereby controlling the copy or reproduction or tracing the false copy on the basis of the detected control information.
Here, embedding of the digital watermark information in the movie film will be considered. In recent years, digitalization is in progress in the process of making a movie film, and generally, some digital transformation such as transforming the film into a digital signal once, editing the digital signal in a studio, and restoring the finished digital signal into a film is performed when preparing movie materials. Therefore, by embedding the digital watermark information in the digital signal in the process of preparing the movie film, the digital watermark information can be embedded in the movie film.
In order to use the digital watermark as a measure to prevent the false copy by private shooting in the theater as described above, it is important to have a resistance against geometry deformation (geometrical deformation occurring in the process from the projection of a movie film having the digital watermark information embedded therein on the screen until the shooting of the movie film by the video camera), and a frame jitter (temporal displacement generated by the individual difference in rotational velocity of the film projector).
Various digital watermark methods are proposed, and one of the methods of providing the digital watermark is a method in which a technology of spectrum diffusion is applied is known. In this method, the watermark information is embedded in the digital moving image in the following procedure.
[Step E1]
Spectrum diffusion is achieved by multiplying an image signal by PN (Pseudorandom Noise) series.
[Step E2]
An image signal after the spectrum diffusion is transformed in frequency (for example, DCT transform).
[Step E3]
Watermark information is embedded by changing the value of a specific frequency component.
[Step E4]
An inverse frequency transform (for example, IDCT transform) is performed.
[Step E5]
Inverse spectrum diffusion is performed (the same PN series as in Step E1 is multiplied).
On the other hand, detection of the watermark information from the digital moving image having the watermark information embedded therein as described above is achieved in the following procedure.
[Step D1]
The image signal is multiplied by the PN series (the same PN series as in Step E1) to achieve the spectrum diffusion.
[Step D2]
The image signal after the spectrum diffusion is transformed in frequency (for example, DCT transform).
[Step D3]
The value of the specific frequency component is targeted and watermark information embedded therein is extracted.
As one of methods of embedding a digital watermark for moving image for embedding an image signal according to the watermark information, a technology to embed a digital watermark signal in an input image signal by generating a digital watermark signal using a specific frequency component signal extracted from the input image signal is proposed, and this method has a strong resistance against the geometry deformation (for example, see Claim 1, FIG. 1 in JP-A-2005-68556 (KOKAI)).
When applying the digital watermark aiming the prevention of the false usage, it is necessary to have a nature which resists loss or alteration of the watermark information resulted from various general operations which are likely applied to the digital copyright production or intended attack applied thereto (robustness). As an attack to disable detection of the watermark information with respect to the digital image having the watermark information embedded therein, there is cutting out of the image, scaling (enlargement/contraction), or the like.
In the related art in which the technology of spectrum diffusion is applied, when the image subjected to the attack as described above is input, the process of estimating the PN series used in Step E1 for embedding is performed when detecting the watermark information to restore the synchronism of the PN series, and the processes in Steps D1 to D3 are performed, so that the embedded watermark information is extracted. However, in order to restore the synchronism of the PN series only from the image signal, it is necessary to perform a search by trying the process with a plurality of candidates and employing the well detected result. Therefore, there is a problem of increase in amount of calculation and the scale of the circuit. Also, since the watermark information is weakened in the attacked image, there is a problem such that even when the cutting out or scaling of image is found and the detection corresponding thereto is tried, the watermark information cannot be detected.
In contrast, a technology of embedding a digital watermark signal on the basis of the specific frequency component signal extracted from the input image signal for embedding and detecting the specific frequency component signal equivalent to the embedded side extracted from the input image signal at the time of detection is known to have a resistance against the geometry deformation, but the embedded position having the resistance against the geometry deformation is limited, and hence it is difficult to embed a plurality of to-be-embedded information at specific positions having resistance against the geometry deformation.